What Is the Original Threshold for This Neuron?
Every time you move a muscle, recall a memory, or even blink, billions of neurons in your brain are sending electrical signals. But when neuroscientists ask, “What is the original threshold for this neuron?” they aren’t just looking for a number. That critical point is called the threshold potential. But these signals don’t fire randomly. Each neuron waits for a precise moment—a critical point—before it decides to “talk” to its neighbors. They are digging into the very foundation of how neurons decide to fire, how that decision can change, and what it reveals about the health and function of the nervous system.
In simple terms, the original threshold refers to the voltage across a neuron’s membrane that must be reached to trigger an action potential under standard, baseline conditions. That's why this is not a fixed, universal value; it varies between neuron types, between species, and even within the same neuron over time. Nonetheless, the concept of an “original” or “classical” threshold—the one described by Hodgkin and Huxley in their landmark 1952 experiments—remains the cornerstone of modern neuroscience education and research Surprisingly effective..
The Concept of Threshold Potential: A Biological Gatekeeper
Neurons maintain a resting membrane potential of approximately –70 mV (millivolts), meaning the inside of the cell is negatively charged relative to the outside. When excitatory inputs from other neurons arrive, they cause small depolarizations (voltage changes toward zero). If these depolarizations are large enough to bring the membrane potential to around –55 mV, the neuron reaches its threshold potential. At that moment, voltage-gated sodium channels open explosively, sodium ions rush in, and the action potential fires—an all-or-none event Nothing fancy..
This threshold is not a mere suggestion; it is a biological gatekeeper. This leads to below threshold, no action potential occurs. At or above threshold, the neuron commits to firing. This binary behavior ensures reliable communication and prevents random noise from triggering false signals.
Worth pausing on this one.
Why "Original" Matters
The word “original” in the question “what is the original threshold for this neuron” carries two meanings:
- Historical meaning: The threshold value first measured in classic experiments (e.g., squid giant axon) that established the foundation of electrophysiology.
- Contextual meaning: The threshold that a particular neuron exhibits under normal physiological conditions, before any modulation or pathology alters it.
For most typical mammalian neurons, the original threshold falls between –55 mV and –50 mV. That said, this is a reference point, not a constant. The true elegance of the threshold concept lies in its dynamic nature.
Scientific Explanation: Ion Channels and the All-or-None Law
To understand the threshold, we must look at the molecular players. Even so, the voltage-gated sodium channels (Nav channels) are the key. They have two important gates: an activation gate (opens quickly upon depolarization) and an inactivation gate (closes slowly after opening). At rest, the activation gate is closed. Even so, when the membrane depolarizes to threshold, the activation gate opens, allowing sodium to enter. This inward current further depolarizes the membrane, opening more Nav channels in a positive feedback loop that generates the rising phase of the action potential The details matter here. That alone is useful..
The threshold voltage is the point at which this regenerative feedback becomes self-sustaining. Below that point, the sodium influx is too small to overcome the potassium leak currents that oppose depolarization. Above that point, the sodium current dominates Still holds up..
The Hodgkin-Huxley Model
The original quantitative description of threshold came from Alan Hodgkin and Andrew Huxley, who worked on the giant axon of the squid. And in their model, this threshold was around –55 mV for the squid axon. They defined the threshold as the membrane potential at which the net inward current (sodium) just exceeds the net outward current (potassium and leak). That value became the classical reference taught in textbooks worldwide.
On the flip side, even in the squid axon, the exact threshold depended on temperature, extracellular ion concentrations, and the rate of depolarization. A rapid depolarization can lower the threshold because sodium channels activate faster than potassium channels, giving sodium a head start. A slow depolarization raises the threshold because potassium channels have time to activate and oppose the depolarization.
Factors That Influence a Neuron’s Original Threshold
No two neurons are identical. The “original threshold” of a given neuron is shaped by several intrinsic and extrinsic factors:
1. Sodium Channel Density and Distribution
Neurons with a high density of voltage-gated sodium channels in the axon initial segment (the site where action potentials typically originate) have a lower (more negative) threshold. The axon initial segment is specialized for precise spike initiation. If channel density is reduced (e.g., in certain neuropathies), the threshold becomes more positive, making the neuron harder to excite.
2. Subthreshold Conductances
Some neurons have persistent sodium currents or low-threshold calcium currents that amplify small depolarizations, effectively lowering the threshold. Others have A-type potassium currents that activate at subthreshold voltages and oppose depolarization, raising the threshold. The balance between these conductances defines the neuron’s excitability And it works..
3. Myelination and Axon Diameter
Myelinated neurons conduct signals rapidly, but the threshold at the nodes of Ranvier depends on the nodal sodium channel density. Larger axons generally have a lower threshold because they have a larger surface area for ion channels, but also a larger capacitance that requires more charge to depolarize.
4. Input Location and Dendritic Integration
The original threshold is measured at the axon initial segment, but the inputs that push the neuron to threshold come from the dendrites. Dendritic morphology, active dendritic spikes, and synaptic strength all influence how much current reaches the trigger zone. This is why the same neuron can have different effective thresholds depending on the pattern of synaptic input Simple as that..
5. Neuromodulation and Plasticity
Neurotransmitters like dopamine, serotonin, and acetylcholine can alter ion channel properties via second messengers. To give you an idea, dopamine can enhance persistent sodium currents in some neurons, lowering the threshold. Conversely, increased potassium channel activity (e.g., via GABAergic inhibition) can raise the threshold. Because of this, the “original” threshold may shift in response to learning, arousal, or disease.
FAQ: Common Questions About Neuronal Threshold
Q: Can a neuron fire below threshold? A: No, by definition. Subthreshold depolarizations cannot trigger the full regenerative sodium current. That said, some neurons show “subthreshold oscillations” that can bring the membrane close to threshold, and small local responses may occur, but not a full action potential.
Q: Does the threshold change from one action potential to the next? A: Yes. After an action potential, the neuron enters a refractory period. During the absolute refractory period, sodium channels are inactivated, making the threshold effectively infinite. During the relative refractory period, the threshold is elevated (more positive) because some potassium channels remain open and sodium channels are still recovering.
Q: What is the difference between threshold potential and resting potential? A: Resting potential is the stable voltage of a neuron at rest (about –70 mV). Threshold potential is the voltage that must be reached to trigger an action potential (about –55 mV). The difference (–15 to –20 mV) is the required depolarization.
Q: How do scientists measure the original threshold? A: Using a technique called patch-clamp electrophysiology in current-clamp mode. The experimenter injects a small current into the neuron and slowly increases it until an action potential fires. The membrane voltage just before the spike is recorded as the threshold Most people skip this — try not to..
Q: Is the original threshold the same for all neurons? A: No. Cortical pyramidal neurons may have a threshold around –55 mV, while cerebellar Purkinje neurons can have a threshold near –60 mV. Sensory neurons like nociceptors (pain fibers) often have a more depolarized threshold (–45 mV). The variation reflects functional specialization Took long enough..
Conclusion: Beyond the Original Number
Understanding “what is the original threshold for this neuron” leads us far beyond a simple voltage number. It opens a window into the layered machinery that governs neuronal excitability, information processing, and brain function. The threshold is not a static wall but a dynamic filter that adjusts based on prior activity, neuromodulation, and the neuron’s molecular anatomy.
For students and researchers alike, the original threshold provides a foundation—a baseline from which we can explore how neurons become hyperexcitable in epilepsy, how they become less responsive in neurodegenerative diseases, and how they adapt during learning. By mastering this foundational concept, you gain the ability to decode the language of the brain, one spike at a time.
Whether you are studying for an exam or designing a computational model, remember: the threshold is not just a number; it is the neuron’s answer to the question, “Is this signal important enough to pass along?” The original threshold, then, is that neuron’s native, unperturbed answer—a reflection of its unique role in the symphony of neural activity.
